Processes that use membranes to separate mixtures of various fluids including gases are accepted applications in many industries. Representative of these processes are microfiltration, ultrafiltration, reverse osmosis, and gas separation. Membranes used to accomplish these separations have been fabricated in various geometries, such as flat sheet, spiral wound flat sheet, tubular, and hollow fibers. The type of membrane shape is usually dictated by the nature of the separation that is to be effected. When performing a separation on a viscous liquid mixture, for instance, it may be advantageous to use a membrane in a large diameter tubular configuration in order to maintain fluid velocity and to minimize fouling of the membrane surface. Conversely, when separating fluids with low viscosities, such as gases, the use of membranes in a hollow fiber configuration is more appropriate.
The fine hollow fiber geometry for membrane fabrication is particularly advantageous because it can yield very high surface area-to-volume ratios. Much of this benefit is derived from the fact that the membrane support structure is integral to the hollow fiber; that is, the membrane is a self-supporting entity. This is in contrast to flat-sheet membranes that are typically cast onto a nonwoven fabric, or to tubular membranes that are frequently cast onto a rigid porous backing tube. Thus, a significant portion of the module volume of flat-sheet, spiralwound, and tubular membranes is consumed by the membrane support structure. This volume is consequently unavailable for packing such modules with additional active membrane area.
Commercially, large bundles of hollow fiber membranes are assembled into permeators or modules. The fibers in these modules are sometimes arranged in a parallel fashion, although it is often advantageous to wind the fibers around a core to impart structural integrity to the bundle. As part of the hollow fiber membrane module fabrication process, at least one end of the fiber bundle is cast or potted in what is commonly referred to as a tubesheet. More commonly, both ends of the bundle are so encapsulated. The tubesheet serves to hold the fibers in a fluid-tight relationship such that the feed fluid is isolated from the permeate fluid, thus allowing components of the fluid to be separated by selective passage of one or more components of the feed mixture through the membrane.
Tubesheets can be fabricated by using any one of a number of liquid resinous materials that subsequently solidify, frequently through a chemical curing process. Typical of such resinous compounds used for casting such tubesheets are thermoset polyurethane and epoxy resins. The liquid resin may be applied to the ends of the hollow fiber bundle by any suitable means. Fritzche et al. in U.S. Pat. No. 4,323,454 describe a process in which a hollow fiber bundle is placed in a mold while a liquid resinous composition of relatively low viscosity is poured into said mold. McLain, in U.S. Pat. No. 3,422,008 makes use of a resin applicator to form the tubesheet in a membrane module in place even as the hollow fibers are being wound into a bundle around a core. It is understood that the physical characteristics of the liquid resin can be chosen to suit the particular method of application.
The properties of the cured resinous composition must meet the demands of the particular application of the membrane module. Thus, the requirements for the tubesheet of a hollow fiber membrane module used for low pressure separation of dissolved solids in liquids may be different from the requirements for the tubesheet of a hollow fiber membrane module used to separate gaseous components at high pressure. With respect to hollow fiber membrane modules used for gas separation, there are several properties of the tubesheet that are commonly desirable. First, the cured resin must be of sufficient strength to withstand the pressure differential across the tubesheet during operation of the module. Frequently, the feed pressure of the gas can be in excess of 80 atmospheres and consequently the differential pressure across the tubesheet will approach this value if the permeate pressure of the membrane approaches atmospheric pressure. In addition, the solidified resinous mixture comprising the tubesheet must also be amenable to being cut or severed in a clean fashion such that the bores of the hollow fibers can be opened to allow free passage of gas along the length of the hollow fibers. Finally, the cured resin must exhibit good flexibility and adhesion to the hollow fibers to maintain a fluid-tight relationship between the hollow fibers and the tubesheet, thus preventing unwanted species in the feed stream from mixing with the permeate.
It is frequently found, however, that meeting all these requirements in a single component tubesheet material is difficult. For instance, cured resinous materials that exhibit high strength commonly possess high glass transition temperatures (T.sub.g) While such intractable materials have the high degree of hardness and tensile strength desired for high pressure operation, their adhesion and flexibility characteristics are generally inferior to softer, lower T.sub.g materials. As suggested above, poor adhesion of the tubesheet material to the hollow fiber can result in undesirable mixing of feed and permeate components during operation of the module. Thus, it is frequently required to make compromises to the characteristics of a single component tubesheet. In most cases the bulk strength and hardness of the tubesheet cannot be compromised in order to avoid catastrophic module failure; thus membrane modules are manufactured with tubesheets that exhibit less than optimal adhesion and flexibility characteristics at the hollow fiber tubesheet interface. These deficiencies of the tubesheet in the interface area can lead to poor membrane module performance.
FIG. 1 depicts this critical area of the hollow fiber membrane module known as the interface for a conventional hollow fiber membrane. Hollow fibers 1 with porous walls 2 are encapsulated in an appropriate resin that forms the tubesheet 5 in a manner such that the hollow fiber bores 3 remain open. The interface region 6 is the area at the boundary between the potted and unposted segments of the hollow fibers in the tubesheet that is distal to the terminal end of the tubesheet 7.
Frequently there is no clear demarcation line at the interface region because of the condition known as wicking. Wicking occurs when the liquid potting compound is drawn up the interstices 4 between the hollow fibers by capillary action to form an irregular resin boundary 6 on the fibers. This wicked portion of the resin can form sharp, hard structures of varying lengths against the bases of the hollow fibers. While the exact mechanism for membrane performance failure due to defects at the interface region is not known, it is believed that this failure is related to the manner in which the modules are operated and the mechanical dissimilarity of the hollow fibers and the potting resin.
Hollow fiber membranes are operated in one of two modes: bore-side feed flow or shell-side feed flow. In the former, the fluid to be processed is fed into the bore side of the fiber and the permeate flows through the membrane walls and into the so-called shell side of the module. In the latter mode of operation, the feed fluid is delivered to the shell side of the membrane and the permeate is collected through the fiber bores. The shell-side feed mode of operation is commonly used in gas separation membrane modules wherein cross-membrane differential pressure is high. When membranes are operated in this mode, the fiber is subject to compression from forces exerted by the feed fluid. Because the hollow fibers are typically composite or asymmetric in structure and constructed from polymeric materials with a large bulk void volume, the fiber can compress under extreme external cross-membrane pressure. This distortion of the fiber's original shape can cause membrane leakage if the membrane is torn away from the rigid tubesheet material. This particular type of defect also can be caused by mechanical vibration or pressure surges that can occur during operating of the module. These defects share the same root cause, however, which is the movement of the flexible, compressible hollow fiber membrane away from the rigid tubesheet material at the module interface.
There have been many processes disclosed in the prior art that attempt to produce a durable leak-free tubesheet for hollow fiber membrane modules. Molthop, in U.S. Pat. No. 4,389,363, cites the use of an elastomeric urethane potting compound for encapsulating hollow fibers in dialysis membrane modules. While such resilient compounds may form a durable tubesheet for low pressure fluid separation applications, they are unsuitable for use in high pressure gas separation modules because they lack the mechanical strength and temperature capability that are required in such processes.
Coplan et al. in U.S. Pat. No. 4,207,192 disclose the application of a ring of rubber cement to a hollow fiber membrane bundle before the tubesheet is formed. This ring of cement acts as a barrier to prevent undue wicking of the potting compound. Because this material is applied prior to potting, there exists the possibility of air gaps between the two material layers. These gaps are sites of unprotected interface that can be accessed by the process fluid should the adhesive bond between the two materials fail.
Hayashida et al. in Japanese Early Disclosure Patent Application Hei4-334529 disclose the application of a silicone protective resin at the interface area of a hollow fiber membrane module. The silicone resin is applied after the fibers have been potted in an epoxy resin tubesheet. It is noted that while the silicone resin is normally placed adjacent to the tubesheet resin, the two layers do not necessarily contact one another. It is also stated that if the gap between the two layers is too large, the effectiveness of the protective layer is diminished.
Cabasso et al. in a report prepared for the Office of Water Research and Technology under Contract Number 14-30-3165, reported on the use of silicon rubber at the potting interface. Cabasso et al. used a silicon rubber cap over the epoxy potted region of the module to reduce epoxy wicking and to minimize membrane failure in this region of the module. It is further reported that the silicon rubber cap did not completely eliminate the failures.
Thus there still exists a need for a hollow fiber potting process that yields high strength and durability to tubesheets that can withstand high differential pressures and maintain a fluid-tight defect-free seal with the fibers under these extreme operating conditions.